Chapter 25 - Radioactivity Flashcards
Define radioactive decay.
The process by which unstable nuclei split, or decay, by emitting ionising radiation.
What is meant by the words ‘spontaneous’ & ‘random’ when describing the nature of decay?
Spontaneous: occurs without external influence.
Random: cannot predict when a nucleus will decay.
What is an alpha particle?
A helium nucleus (two protons & two neutrons).
What is a beta particle?
A fast-moving electron.
What is a gamma ray?
High energy photons of wavelength <10¹³.
What is the range of alpha radiation?
Short, absorbed by a few cm of air or by a sheet of paper.
What is the range of beta radiation?
~1m in air and 1-3mm in aluminium.
What is needed to absorb gamma radiation?
A few cm of lead or several m of concrete.
Arrange alpha, beta & gamma radiation from most to least ionising.
Alpha, beta, gamma.
Define activity of a source.
The rate at which nuclei decay or disintegrate in a radioactive source, measured in becquerels (Bq) or decays per second.
Define decay constant.
The probability of decay of an individual nucleus per unit time.
Describe the process of carbon dating.
-The ratio of carbon 12 to 14 atoms for a dead sample of material is compared to that of a living one.
-A material with more carbon 12 atoms is older.
State some limitations of carbon dating.
-Assumes ratio of carbon 12 to 14 in the atmosphere has remained constant, though it may have changed due to volcanic eruptions, burning fossil fuels etc.
-Tiny amounts of carbon 14 in organisms are difficult to measure accurately.
-Cannot be used to date rocks.
Outline an experiment that can be used to investigate the absorption of radiation.
-Place radioactive source in front of Geiger-Muller tube.
-Place an absorber between them.
-Vary thickness of absorber and see how this affects count rate.
When using a Gieger counter, how do you remove background count rate from the measured count rate?
Measure the background count rate and subtract this from the measured count rate of the source.
Define half-life.
The amount of time it takes for the activity of a source to half.
Outline an experiment to determine half life.
-Measure the count rate of a source every 30s.
-Plot a graph of ln(count rate) against time.
- Gradient = ln2/t½
